Thermal display unit

ABSTRACT

This invention relates to improvements in display units, and it relates particularly to units in which the display utilizes temperature differences and principally where the temperature difference are made to provide a visual display. The specification discloses a rectangular coordinate printer and plotter incorporating a matrix of semiconductor rectifier junctions connected in rows and columns whereby selected ones of the rectifiers may be rendered conductive. Heat generated in the rectifier is conducted to the surface of a platen where it acts upon a thermally sensitive paper which changes color at the point of heat application. Two forms of platens are illustrated and described. In one the several rectifiers of the matrix comprise discrete units interconnected by conductor deposits and wires. In the other, the several rectifiers are formed as a monolithic structure of semiconductor material.

7 United States Patent [72] Inventors Raymond Davis 13942 Brenan Way,Santa Ana, Calif.

92705; Charles K. Krill, 3030 Aha Laguna Blvd., Laguna Beach, Calif.92651 [21] Appl. No. 794,029

[22] Filed Jan. 27, 1969 [45] Patented Sept. 28, 1971 [54] THERMALDISPLAY UNIT 7 Claims, 9 Drawing Figs.

[52] U.S. Cl 340/166,

219/216, 340/324 [51] Int. Cl H051) 3/20 [50] Field of Search 340/166,

1,549,666 12/1967 France ABSTRACT: This invention relates toimprovements in display units, and it relates particularly to units inwhich the display utilizes temperature differences and principally wherethe temperature difference are made to provide a visual display. Thespecification discloses a rectangular coordinate printer and plotterincorporating a matrix of semiconductor rectifier junctions connected inrows and columns whereby selected ones of the rectifiers may be renderedconductive. Heat generated in the rectifier is conducted to the surfaceof a platen where it acts upon a thermally sensitive paper which changescolor at the point of heat application. Two forms of platens areillustrated and described. In one the several rectifiers of the matrixcomprise discrete units interconnected by conductor deposits and wires.In the other, the several rectifiers are formed as a monolithicstructure of semiconductor material.

THERMAL-DISPLAY UNIT This invention relates to improvements in thermaldisplay units and in methods of making them. It relates particularly tounits in whichinformation represented by electrical signals is presentedin the form of thermal signals whose spacial relation is indicative ofthe information represented electrically.

A number of devices exist whichpresent infonnation contained inelectrical signals in the form of spacially arranged indicia. Familiarexamples are the matrix printer and various devices which displaysymbols, numeral and letters appearing as the composite of a number ofelemental indicia. An object of the invention is to provide an improveddevice of this kind. Another object is to provide an apparatus whichwill produce a therrnalsignal at any of a number of predefined spacialpositions in response to electrical signals in accordance with aprearranged code.

The invention is applicable where it is desired to represent infonnationby location or relative position of display signals on either flat ornonflat surfaces. It is applicable to the presentation of information inpositions identified by polar coordinates or rectangular coordinates orby any other system of coordinates into which a plane or surface may bedivided. Thus information can be presented in rows or columns which mayor may not be straight and it is an object of the invention to providean apparatus which can present information thermally in any of the widevariety of spacial arrangements so defined. Nonetheless, the inventionis particularly well suited to the presentation of indicia in straightrows and columns as in conventional X-Y plotting and an object of theinvention is to provide a superior X-Y plotting apparatus.

Another object of the invention is to provide a unique and efiicientstructure for the conversion of information represented by electricalsignals to spacially arranged indicia by transforming the electricalsignals to thermal signals.

Certain of these, and other objects and advantages of the invention, arerealized by the provision of a thermal display unit utilizing heatgenerated at selected spaced points by an electrically conductive devicewhich experiences greater heat rise as an incident to current flow inone direction than in the other.

In the drawings:

FIG. 1 is an isometric view of a glass block in which heat conductorsare imbedded and from which platens are cut;

FIG. 2 is a top plan view of a platen according to the invention;

FIG. 3 is a bottom plan view of the platen of FIG. 2;

FIG. 4 is a cross-sectional view of a portion of the platen taken online 4-4 of FIG. 3;

FIG. 5 is a cross-sectional view of a portion of the platen taken online 5-5 of FIG. 3;

. FIG. 6 is a schematic diagram of a portion of the platen shown inFIGS. 2, 3, 4 and 5 illustrating connection to x and Y-selectorswitching units;

FIG. 7 is an isometric view of a monolithic block of semiconductormaterial as it appears in a preliminary stage of construction of analternative form of platen embodying the invention;

FIG. 8 is a completed platen of alternative design which embodies theinvention and incorporates the monolithic structure of FIG. 7; and

FIG. 9 is an isometric view of a fragment of an X-Y plotter showing aplaten holder, a platen embodying the invention, and a thermosensitiveprinting paper overlying the platen.

It enough rectifiers are employed, it is possible to energize selectedones of a number of two terminal electrical elements with a minimumnumber of switches by dividing the terminals into groups. Correspondingterminals of the elements are divided into groups and each group isconnected to a switch. The other terminal of each element is assigned toone of the second series of groups such that no two elements which arein the same group of the first series are classified in the same groupof the second series. Each of the second groups also is connected to aswitch. Thus arranged, the elements can beenergized selectively withoutenergization of any other element if a unidirectionally conductingdevice is connected series with it. The invention makes use of thisphenomenon except that it omits the two terminal elements.

Such a circuit, a circuit of the kind employed in the invention, isillustrated schematically in FIG. 6. The X-selector l0 and theY-selector 12 are switch units selectively energizing one or more of theoutput lines emanating from them from a course of electrical power, notshown. Thus the X-selector I0 is effective to energize line 13, 14 or 15or any combination of them. The X-selector may comprise a steppingswitch or a solid state switch bank or any of a number of known devices.The Y-selector l2 similarly may comprise any of such wellknown deviceswhich will energize one or more of the output lines 17, 18 and 19. Theunidirectionally conductive device interconnects the several lines ateach intersection. Thus rectifier 20 interconnects lines 13 and 17. Therectifier 21 interconnects lines 14 and 17 and rectifier 22interconnects lines 16 and 17. Rectifiers 23, 24 and 25 interconnectline 18 with lines l3, l4 and 16 respectively. Similarly, three otherrectifiers interconnect line 19 with each of the lines l3, l4 and 16. Ifline 16 is energized by the X-selector 10 and line 17 is energized bythe Y-selector 12, then a current will flow through rectifier 22;provided, of course, that the polarity of the currents applied to the Xand Y-selector inputs is proper as shown. Every other rectifier in thenetwork is effective to prevent connection of the line 16 with any otherof the lines emanating from the Y-selector. Similarly, all of therectifiers acting together prevent interconnection between the lines 17and any line of the X-selector other than line 16. If lines 14 and 16are energized by the X-selector and line 17 is energized by theY-selector then current will flow through two of the rectifiers numbered21 and 22. But current may flow through no other rectifier unless it beconnected to one line energized by the Y-selector and one line energizedby the X-selector. The several rectifiers may be of any well-known type.Thus the may comprise metal oxide rectifiers or crystal rectifiers orany other type of rectifier in which losses within the rectifier resultin its becoming heated. Advantageously the rectifiers employed in theinvention comprise semiconductor materials. In the preferred form theycomprise a junction and while that junction may comprise one element ofa transistor or other, more sophisticated multiple terminal device, inpreferred form the rectifier comprises a single, P N, semiconductorjunction.

The representation in FIG. 6 is schematic. The rectifier devices neednot be arranged in rows and columns as they are there shown to bealthough the row and column arrangement is quite advantageous and hasbeen selected for illustration in the several embodiments of theinvention depicted in the drawings. As the X and Y-selectors areactuated to complete the circuits to any one of the rectifiers thatrectifier becomes heated. The invention envisions the incorporation ofmeans for detecting which ones, if any, of the rectifiers are heated.The rectifiers are given a placement (or a heat conductor connected toconduct heat from the rectifier to another place is given a placement),relative to all'the other rectifiers, which has a predefined meaning.That is, the spacial arrangement of the heated rectifiers is given somemeaning in accordance with a predefined convention or code. For example,if a number of rectifiers are arranged in a matrix of parallel rows andcolumns then selected rectifiers may be heated by connection to anelectrical power source so that the heated rectifiers form a curve inrectangular coordinates or so that they form various symbols such, forexample, as letters and numerals as illustrated in FIG. 9. The inventioncontemplates the employment of means for converting to visible form thethennal signals and displays thus established. This can be done in avariety of ways. Advantageously it is done by utilizing the heat fromthe rectifiers to change the color of a thermosensitive paper at thepoint of heat application. In FIG. 9 the layer of paper 26 whichoverlies the platen 27 is thermosensitive in that it changes color whensubjected to heat. In FIG. 9 a number of points in platen 27 have beenheated as an incident to current flow through rectifiers arranged in arectangular coordinate pattern as in FIG. 6. These rectifiers haveburned or printed" the letter A" and the figure 7 on the thermallysensitive printout paper as shown in FIG. 9.

The platen 27 is similar in construction to the platen illustrated inFIGS. 2, 3, 4 and 5, but the latter is designated by the referencenumeral 28. The top of this platen is illustrated in FIG. 2. Here amatrix of circular heat-conducting elements are exposed at the uppersurface of a sheet of glass. All of the heat conductors of a horizontalrow are interconnected by a layer of electrically conductive materialdeposited in a groove which extends the width of the glass sheet. Theglass sheet is designated by the reference numeral 30. Representativeones of the heat conducting elements are designated by the referencenumeral 32. Representative sections of the conductive material depositedbetween the heat conductive elements are designated by the referencenumeral 34. The circular elements 36 which comprise the lower row ofcircular elements in FIG. 2 are electrical connector terminals.

Each of the heat-conducting elements 32 seen in FIG. 2 is in electricalcommunication with a rectifier at its opposite face as illustrated inFIG. 3. Each of the rectifiers 38 comprises a die of germanium doped toform a P N junction. The heat conductors 32 are also electricalconductors and one material of each junction, advantageously the Pmaterial, is bonded to a respectively associated one of the heatconductors 32. FIG. 3 is a view looking down on the N material side ofthe rectifiers. The rectifiers of each column are bonded together attheir N surfaces and they are connected to a respectively associated oneof the terminals 36. For identification, several of the wires which bondthe columns together are designated by the reference numeral 40.

Referring to FIG. 4 where the platen is shown in cross section, the heatconducting elements 32 comprise short lengths of round wire embedded inthe flat sheet of glass 30 so that their respective axes are paralleland so that the spacing between the several heat-conducting elements isuniform. The connector terminal 36 also is formed of a short length ofthe same wire and differs from the'heat conducting elements, in thisembodiment, in its purpose rather than its construction. The material ofthese wires is advantageously a good conductor of heat. Advantageously,it is also a good conductor in electricity for the invention does notrely upon the resistance of the heat conductors for successful practice.The P material has greater resistivity than does the N material of thesemiconductor die. Most of the heat generated in the semiconductormaterial is generated at the P side. Advantageously, it is this sidewhich is bonded to the heat conductor. The material may be formed byinclusion of aluminum atoms as impurities in the germanium of the die.It follows that aluminum, being also a good conductor of heat, makes anexcellent heat conducting element although other materials will be foundto be suitable. The N side of the several semiconductor dice 38 areinterconnected by a wire 40 which is also bonded to the terminal member36.

A cross-sectional view of the platen taken at right angles to the viewdepicted in FIG. 4 is shown in FIG. 5. A channel is etched in the uppersurface of the glass platen at each row of heat-conducting elements asshown in FIG. 2. A layer 34 is conducting material is deposited in thatchannel so that it makes electrical contact with all of the heatconducting elements 32.

It will be apparent that the heat conductors need not be arranged sothat their N faces all lie in a flat plane. It is entirely possible toproduce platens that are cylindrical or have various other curvedshapes. It will also be apparent that the heat-conducting elements neednot be arranged in rows and columns that are parallel. For example, itwill be apparent that the rows could be formed as concentric circles,that the columns could be arranged to radiate from the center of thoseconcentric circles whereby to form a platen whose elements are arrangedin polar coordinate form. It will be apparent that more than onerectifier can be connected in series or in parallel across a junction ofenergizing conductors and that the rectifiers may be placed at spacedpoints to form various patterns ultimately to provide individual visibledisplays different from others. It will be apparent that a wide varietyof coordinate arrangements and alphabetical and numerical and othersymbols may be provided by appropriate arrangement of the rectifiers intheir heat-conducting elements.

To produce a platen, a sheet of electrically and thermally insulatingmaterial is formed. That sheet if provided with a plurality ofheat-conducting elements which extend to the op posite faces of thesheet and are arranged in some predefined pattern by being appropriatelyspaced in the body of the sheet. FIG. 1 illustrates that such a sheetmay be formed by extending a series of wires of suitable material, hereKovar, arranged taut and parallel and spaced in rows and columns betweentwo end holders 42 and 44. The space between the holders 44 is filledwith molten glass which is permitted to harden into a solid block. Theblock is then sliced in the direction perpendicular to the axis of thewires. Two such slices, identified by the reference numerals 45 and 46respectively, have been sliced from the block of glass 47 in FIG. 1.After slicing the faces of the flat sheet are ground parallel. One faceof the sheet is covered with a resist" material except along linesmarking rows of the Kovar heat conductor wires. The wire ends and theremaining surface of the glass sheet are protected by the resist from anacid which is then used to etch a channel in the surface of the sheetbetween the aluminum wires of each row. Conducting material,advantageously a very thin coating of highly electrically conductivematerial, here copper, is deposited in the channel thus formed. Thedeposited layer of copper is made to contact each of the Kovar wire heatconductor elements of the row.

At the opposite face of the sheet semiconductor dice are connected tothe flat ends of the conducting wires. Each dies is bonded to arespectively associated heat conducting element using heat and pressureto accomplish a mechanical and electrical bond. All of the dice areoriented in the same direction, advantageously so that their P materialis toward the heat-conducting element. Thereafter, the other side of allthe semiconductor rectifiers of each column are bonded together in awire bonding process, or other interconnection process, whereby all ofthe semiconductor rectifiers are connected in the matrix in a fashionthat is described electrically by the schematic diagram of FIG. 6.

The platen thus formed is made of readily available, reliable and longlasting material. If added protection is desired, the upper surface ofthe platen is easily passivated and the lower surface is readilyprotected by coatings that are impervious to air and moisture. Thesemiconductor dice need not be of high quality. In fact, rectifiers ofpoor quality in that they have high resistive loss, are preferred.

Procedures for embedding the conductor wires in glass, for slicingglass, for bonding semiconductors to wires imbedded in glass, foretching glass, and metallizing etched surfaces, for wire bonding, andall of the other steps involved in the process are known and availablethereby permitting the production of relatively large sized, glassfilled platens.

Moreover, techniques are well known by which monolithic semiconductorplaten structures may be produced. It is possible to produce P-typesemiconductor material in very thin sheets of relatively large area. Ifone face of such a sheet is coated with an etch resistant material,except in parallel strips corresponding to evenly spaced rows and theother face of the sheet is covered with an etch resistant materialexcept along lines evenly spaced in the direction of columns and if thewhole sheet is then subjected to an etching material, it is possible toetch the P-material to a sufficient depth so that the sheet isperforated entirely through at those areas at which etching proceedsfrom both sides thereby forming a lattice structure in the form of barsand columns of P material. A structure of this kind is shown in FIG. 7.It is monolithic consisting of three rows 50,52 and 54 and threecolumns, 56, 58 and 60. The numeral 62 designates one of the openingsextending entirely through the semiconductor sheet where etchingproceeding from the opposite side of the plate has removed all of thematerial and formed the through opening. A metal electrode is depositedon one face of each of the rows and the lower face of each of thecolumns. The conductor layer on row 50 is designated by the referencenumeral 63 and the layers on rows 52 and 54 are designated 64 and 65respectively. These layers are deposited on the side of the row ratherthan on the upper face. A layer of metallic conductor material 66underlies the column 56. Layers 67 and 68 underlie the columns 58 and 60respectively. This monolithic structure is only a few thousandths of aninch thick and requires support. In FIG. 8 the monolithic structure ofP-material is shown resting upon a base 70 whose dimensions, at least inone direction, are greater than those of the monolithic row and columnstructure. The upper surface of the base 70 is provided with a depositedlayer of electrically conductive metal in a position to underlie theconductive layer on the other side of the columns of semiconductormaterial. Thus the layer 71 underlies the conductive layer 66.Similarly, the layers 72 and 73 underlie and extend beyond theconductive layers 67 and 68. These several pairs of layers are bondedtogether so that contact may be made with the conductor underlying thecolumns by making electrical connection to the layers 71, 72 and 73.Spaced areas of each of the three semiconductor rows of the monolithicrow and column structure are converted to N- material by a diffusionprocess. The area of each row which overlies a column has doner atomsdiffused into it so that the N-layer extends downwardly to the column ofP-material and entirely across the width of the column. The N-materialboundaries are identified in FIG. 8 by the dashed lines. All three rowsare treated alike. This being a three by three matrix, there are ninesections of N-material. In the rear bar 50 there is a section of N-material 80 at the right, a section of N- material 81 at the center, anda section of N-material 82 at the left. The two sections 80 and 81 areseparated by a segment of P-material 83 and the N-sections 81 and 82 areseparated by a segment of P-material 84. These intervening sections ofP- material cooperate with the N-material sections 81, 82 and 83 in away that isolates each N-material section from the other through a pairof back-to-back diodes in the equivalent circuit. Since these diodesserve to isolate adjacent diode sections of the monolithic structure,the electrical nature of the whole is adequately described by simplifiedequivalent circuit which is shown in FIG. 6. The construction of FIG. 8has the advantage that heat generated by all of the diodes is availablein a single plane whereby it is unnecessary to employ heat conductingelements to create a platen surface from which the heat is transformedto a display. In FIG. 8 the spaces between the several rows would befilled with silicon oxide or other convenient insulating material toprovide a fiat platen surface. The silicon oxide has been omitted fromFIG. 8 for the sake of clarity.

In operation of platens of the kind described above, heat is generatedin the several diodes very rapidly upon being energized and is veryrapidly dissipated in the thermal sensitive paper whereby high speedprinting can be achieved. It will be apparent that current is made toflow through the heat conducting wires 32 in the embodiment of FIGS. 2,3, 4 and 5. To the extent that these wires have resistance this currentflow results in heating. However, the heating is negligible. Thematerial selected for use as heat conductors materials will have verylow resistivity whereby heating results entirely, in practical effect,from heating in the rectifying device. Accordingly, it is entirelypractical to form the conductor which connects to the side of therectifier adjacent to the heat conductors at the rectifier side of thoseconductors. In the embodiment shown, this may be done by depositing theconductor segments 34 on the lower face of the glass plate rather thanto the upper face. In certain applications of the invention thismodification is the preferred form.

Although I have shown and described certain specific embodiments of myinvention, I am fully aware that many modifications thereof arepossible. My invention, therefore, is not to be restricted exceptInsofar as is necessitated by the pnor art and by the spirit of theappended claims.

We claim:

1. A printing heat comprising a heat conductor imbedded in anelectrically and thermally insulating plate with portions thereofexposed in heat conducting relation at opposite sides of said plate, anda two terminal electrical rectifier having impedance resulting ingreater heating when subjected to current flow in one direction than inthe other, said rectifier having thermally conductive connection to saidheat conductor and further comprising means for connecting saidrectifier to a source of electrical power.

2. In a thermal printer:

at least two pairs of conductors ant at least four unidirectional,semiconductor junction devices of a kind whose temperature increaseswhen conducting electrical currents;

a respectively associated one of said devices being connected from eachof said first pair of conductors to each of said second pair ofconductors, said conductors and said devices forming part of a platenhaving a printing surface;

heat-conducting means for selectively conducting heat from each of saiddevices to a respectively associated surface area of said platen.

3. The invention defined in claim 2 in which said heat-conducting meanscomprises a portion of the material of said semiconductor devices.

4. The invention defined in claim 2 in which said semiconductor junctiondevices are formed integrally from a monolithic block of semiconductormaterial.

5. The invention defined in claim 2 in which said platen is formed by anintegral grid of parallel bars of P-type material overlying parallelbars of N-type material.

6. The invention defined in claim 2 in which said heat-conducting meanscomprises a layer of thermal and electrical insulating material and aplurality of spaced heat conductors extending through the layer, eachheat conductor being in thermal communication with a respectivelyassociated one of said devices.

7. A monolithic printing platen comprising an integral grid consistingof a first set of parallel members of P-type material overlying a secondset of parallel members of material extending crosswise of said firstset and including N-type material at the points of intersection of themembers.

2. In a thermal printer: at least two pairs of conductors and at leastfour unidirectional, semiconductor junction devices of a kind whosetemperature increases when conducting electrical currents; arespectively associated one of said devices being connected from each ofsaid first pair of conductors to each of said second pair of conductors,said conductors and said devices forming part of a platen having aprinting surface; heat-conducting means for selectively conducting heatfrom each of said devices to a respectively associated surface area ofsaid platen.
 3. The invention defined in claim 2 in which saidheat-conducting means comprises a portion of the material of saidsemiconductor devices.
 4. The invention defined in claim 2 in which saidsemiconductor junction devices are formed integrally from a monolithicblock of semiconductor material.
 5. The invention defined in claim 2 inwhich said platen is formed by an integral grid of parallel bars ofP-type material overlying parallel bars of N-type material.
 6. Theinvention defined in claim 2 in which said heat-conducting meanscomprises a layer of thermal and electrical insulating material and aplurality of spaced heat conductors extending through the layer, eachheat conductor being in thermal communication with a respectivelyassociated one of said devices.
 7. A monolithic printing platencomprising an integral grid consisting of a first set of parallelmembers of P-type material overlying a second set of parallel members ofmaterial extending crosswise of said first set and including N-typematerial at the points of intersection of the members.